The present invention relates to a rotary-vane pump, particularly for servo steering systems in motor vehicles, which contains a flow control valve and a throttling means.
The function of such pumps is to provide a delivery flow which is as constant as possible despite varying driving speeds of the pump.
A pump of this kind is disclosed in U.S. patent application Ser. No. 07/572,894 filed on Aug. 24, 1990, now U.S. Pat. No. 5,098,259. The difference in the pressure upstream and downstream of a throttling bore is used to operate the flow control valve. With increasing vehicle speed, and consequently with increasing rotational speed of the pump, the flow control valve is opened further, so that a larger proportion of the delivery is passed to the drain side of the pump and the useful flow remains constant.
In power assisted steering systems for high speed vehicles there is an increasing demand that with rising speed of travel the steering reaction should increase. This kind of reaction is intended to give the driver of the vehicle a reliable road feel even at high speeds. However, the increased delivery power of the pump at higher rotational speeds increases the hydraulic steering assistance, and the proportion of mechanical power to be supplied by the driver is reduced.
Another pump is known from GB Patent No. 1,459,161. In this known pump, a throttling means is constituted by a discharge and throttle port in a cylindrical insert which is angularly adjustably disposed in a pressure passage transversely to the axis thereof between a pressure chamber of the pump and a flow control valve.
The invention aims at providing a rotary-vane pump whose delivery flow characteristic is suitable for application to the power steering system.